PEER has just published Report No. 2014/04 titled “Effect of Vertical Acceleration on Shear Strength of Reinforced Concrete Columns” as a new addition to the PEER Report Series. It was authored by Hyerin Lee and Khalid M. Mosalam.

Abstract:
The effect of vertical excitation on shear strength of reinforced concrete (RC) columns has been investigated by various researchers. Field evidence, analytical studies, and static or hybrid simulations suggested that excessive tension or tensile strain of the column may lead to shear degradation, and that vertical excitation can be one of the causes of shear failure. Due to limitations of testing facilities, published literature has not reported the results of dynamic experiments to investigate the effect of vertical excitation on the shear strength of RC columns. Considering that current seismic codes do not have a consensus on the effect of vertical acceleration on the shear demand and capacity, the presented dynamic tests and accompanying analytical investigation contribute to better understanding of the effect of vertical excitation on shear failure, one of the most critical brittle failure mechanisms.

This report provides both experimental and computational results, which confirm that vertical acceleration can induce shear strength degradation of RC columns. Dynamic tests of two reduced geometrical scale specimens were conducted on the University of California, Berkeley shaking table at Richmond Field Station. The two specimens had different transverse reinforcement ratio. As a result of an analytical investigation and preliminary fidelity tests, the 1994 Northridge earthquake acceleration recorded at the Pacoima Dam was selected as an input motion among the 3551 earthquake acceleration records in the PEER NGA database. The chosen ground motion was applied to the test specimens at various levels ranging from 5% to 125%. The specimens were subjected to combinations of vertical component and the larger of the two horizontal components of the selected ground motion record. For the 125%-scale, not only was the combined vertical and horizontal motion applied but also a single horizontal component was considered for direct evaluation of the effect of the vertical excitation.

The experimental results imply that vertical acceleration has the potential to degrade the shear capacity of RC columns. The peak shear force in the 125%-scale run with only the horizontal component was larger than that in the 125%-scale runs with the horizontal and vertical components for each specimen, where the peak force was determined by the shear strength at these high-level tests. For these runs, considerable tensile forces were induced on the tested columns due to the vertical excitation. Tension in the columns resulted in degradation of the shear strength, which is mainly due to the degradation of the concrete contribution to the shear strength. Flexural damage at the top of the column took place before the flexural damage at the base since the bending moment at the top was larger. This was a result of the large mass moment of inertia and rigid body rotation of the mass blocks at the top of the column. In addition, comparison of the bending moment histories at the base and top of the two test specimens indicated that they were opposite in sign during the strong part of the excitation of all the intensity levels, suggesting that the columns were in double-curvature. As a result of flexural yielding at the top and base of the column when bending in double curvature, the shear force reached its shear capacity, which would not take place if yielding occurred only at the base. Consequently, shear cracks occurred and extended over the entire column height as the intensity increased, especially when subjected to significant axial tension.

The analytical investigation also revealed that considerable axial tension forces can be induced in RC columns, which resulted in degradation in the shear strength. Two types of computational models were utilized in the computational platform OpenSees. Models A and B had a beam with hinges element and a nonlinear beam-column element, respectively. In addition, a new shear spring element was implemented in the same computational platform to employ code-based shear strength estimation. The element incorporates the shear strength estimations based on ACI or Caltrans SDC equations, addressing the effect of column axial load and displacement ductility. Both Models A and B were developed without and with the newly-developed shear spring element. Upon improved modeling, results from the analysis of the tested specimens were examined in terms of shear strength variation. Accordingly, current code equations and the corresponding computational models were evaluated. The models without the shear springs did not capture the shear strength degradation accurately, whereas those including the ACI and Caltrans SDC shear springs captured the shear strength degradation due to the axial tension. Both of the ACI and Caltrans SDC springs provided results on the conservative side, where the ACI shear spring predictions were closer to the experimental results than those of the Caltrans SDC shear spring. Elimination of the concrete contribution to the shear strength under any tension was the main reason for the highly conservative predictions of the Caltrans SDC shear strength equation, where the strength reduction caused by ductility was not as significant as that caused by axial tension force.